Materials and process for reversible adsorption of carbon dioxide

ABSTRACT

A solid material is disclosed for reversibly adsorbing carbon dioxide. The material comprises a porous carrier material. Deposited on the carrier material is a salt that is capable of reacting with carbon dioxide. Optionally the solid material further contains a particulate, water-insoluble inorganic material. 
     The material can be used in a process for recovering carbon dioxide from a carbon dioxide containing gas mixture. The gas mixture can be atmospheric air. The process comprises a temperature swing between an adsorption temperature T 1  and a desorption temperature T 2 . The difference between T 2  and T 1  is less than 200 ° C.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates generally to the reversible adsorption of carbon dioxide from a gas mixture, and more particularly to the reversible adsorption of carbon dioxide from ambient air.

2. Description of the Related Art

Carbon dioxide has been identified as a major contributor to climate change. Significant efforts are being made towards the development of adsorbent materials that adsorb carbon dioxide from gas mixtures, such as flue gases, that are relatively carbon dioxide rich. The focus is on adsorbent materials that irreversibly capture the carbon dioxide, for example calcium oxide. By sequestering the captured carbon dioxide in, for example, landfills or abandoned mines in the form of calcium carbonate, the amount of carbon dioxide in the atmosphere is effectively reduced.

In principle capture of carbon dioxide in the form of calcium carbonate is a reversible process, in that carbon dioxide can be released from calcium carbonate by heating. This is a very old process that has been practiced for millennia in lime kilns. However, the decomposition of calcium carbonate requires very high temperatures, in excess of 900° C. For the purpose of carbon dioxide sequestration its capture in calcium carbonate can be considered irreversible in that no captured carbon dioxide can be expected to be released during storage.

It is desirable to reversibly capture carbon dioxide so that it can be released on demand, for use in chemical reactions such as the reverse water gas shift and the production of methanol. For the present purpose carbon dioxide capture is considered reversible if the release of captured carbon dioxide can be accomplished at temperatures below 300° C. For economic reasons it is desirable for the desorption of carbon dioxide to take place at even lower temperatures, for example less than 200° C.

It is also desirable to reversibly capture carbon dioxide from ambient air. Although, compared to flue gases, ambient air has a low carbon dioxide concentration, the temperature of ambient air is much lower, and unlike flue gas ambient air is substantially free of corrosive contaminants.

Thus, there is a need for materials and processes for reversibly adsorbing carbon dioxide from gas mixtures. There is a particular need for reversibly adsorbing carbon dioxide from ambient air.

BRIEF SUMMARY OF THE INVENTION

The present invention addresses these problems by providing a solid material capable of reversibly adsorbing carbon dioxide, said solid material comprising a porous carrier material having deposited thereon: (i) a salt capable of reacting with carbon dioxide; and optionally (ii) a particulate, water-insoluble inorganic material.

Another aspect of the invention comprises a process for recovering carbon dioxide from a carbon dioxide containing gas mixture comprising (a) at a first temperature T₁, contacting the carbon dioxide containing gas mixture with a solid material capable of reversibly adsorbing carbon dioxide; and (b) desorbing at least part of the adsorbed carbon dioxide from the solid material at a temperature T₂, such that T₂>T₁ and ΔT, defined, as T₂ minus T₁ is less than 200° C.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of a test set-up for determining the carbon dioxide absorption properties of solid materials.

FIG. 2 is a graph plotting the carbon dioxide desorption rates as a function of temperature for various solid materials tested in the set-up of FIG. 1.

FIG. 3 is a representation of carbon dioxide loadings of various solid materials tested in the set-up of FIG. 1.

FIG. 4 is a schematic representation of an absorption/desorption device for carbon dioxide and water.

FIG. 5 shows the device of FIG. 4 in its absorption and desorption modes.

FIG. 6 shows the desorption of carbon dioxide from a solid material, as a function of the desorption temperature.

DETAILED DESCRIPTION OF THE INVENTION

The following is a detailed description of the invention.

Definition

The term “reversibly adsorbing carbon dioxide” as used herein means adsorption of carbon dioxide to a material that releases the adsorbed carbon dioxide at a temperature below 200° C.

In its broadest aspect the present invention relates to a solid material capable of reversibly adsorbing carbon dioxide, said solid material comprising a porous carrier material having deposited thereon: (i) a salt capable of reacting with carbon dioxide; and optionally (ii) a particulate, water-insoluble inorganic material.

The temperatures of adsorption and desorption are very important for the economics of the process. The solid material is preferably used in a temperature swing reaction. Large temperature swings require large energy inputs. In addition, large temperature swings tend to cause deterioration of adsorbent materials. Preferably the solid material is capable of adsorbing carbon dioxide at a first temperature T₁ and of desorbing carbon dioxide at a second temperature T₂, such that T₂>T₁ and ΔT, defined, as T₂ minus T₁ is less than 200° C. Preferred are solid materials characterized by a ΔT of less than 160° C.

Preferred are solid materials wherein the salt is capable of reacting with carbon dioxide and water to form a bicarbonate. Preferred are salts of alkali metals and alkaline earth metals, in particular salts of Li, Na, K, Ca, Ba, and mixtures thereof. Examples of suitable salts include carbonates.

Potassium carbonate has been found particularly effective.

The equation for the adsorption reaction on potassium carbonate is given by (1):

L₂CO₃+H₂O+CO₂→2 KHCO₂  (1)

The equation for the desorption step is given by (2):

2 KHCO₃→K₂CO₃+H₂O+CO₂  (2)

Equations (1) and (2) illustrate an important feature of this embodiment of the invention. The adsorption reaction consumes water, which is released during the desorption reaction. Water needed for the adsorption reaction is typically abundantly present in the gas mixture from which carbon dioxide is being adsorbed. Flue gases are, for example, produces by burning a fossil fuel of the general formula C_(n)H_(2n+2), the flue gas contains carbon dioxide and water in close to a 1:1 molar ratio. Ambient air of 25° C. and 50% relative humidity contains about 9 g/kg (=0.5 mole/kg) water, and 400 ppm (or about 0.01 mole/kg) carbon dioxide. Cold, dry air of 5° C. and 25% relative humidity contains about 0.15 mole/kg water. In all three cases there is more than enough water in the gas mixture for the adsorption reaction. In fact, it may be necessary to pre-dry the gas mixture prior to contacting it with the solid adsorbent material.

As shown in equation (2), water adsorbed from the gas mixture (flue gas or ambient air) is released during the desorption reaction. Thus, the adsorption/desorption process also provides water, which may be used in a subsequent CO₂ conversion reaction, or may be used in agriculture, for household use in washing and cleaning, or as a source of potable water.

The porous carrier material may be a honeycomb monolith material, for example of the kind as is used in catalytic converters for the treatment of exhaust gases of internal combustion engines. The carrier may be made of a ceramic material, such as codierite; of a zeolite material; activated carbon; and the like. In another embodiment the porous carrier material is made of a ceramic foam.

Particularly preferred for use as the porous carrier material is activated carbon having a specific surface area in the range of from 150 m²/g to 600 m²/g.

Optionally the solid material contains, in addition to the porous carrier material and the reactive salt, a particulate, water-insoluble inorganic material. In an embodiment this material is an inorganic oxide having a specific surface area of less than 100 m²/g. The material may be derived from a corresponding material having a specific surface area in excess of 100 m²/g by calcination or steam calcination. Examples of suitable materials include alumina, silica, titania, zirconia, ceria, clay, zeolite, layered hydroxide material, hydrotalcite, and mixtures thereof. A particularly preferred example is titania.

Another aspect of the present invention is a process for recovering carbon dioxide from a carbon dioxide containing gas mixture comprising (a) at a first temperature T₁, contacting the carbon dioxide containing gas mixture with a solid material capable of reversibly adsorbing carbon dioxide according to any one of the preceding claims; and (b) desorbing carbon dioxide from the solid material at least in part at a temperature T₂, such that T₂>T₁ and ΔT, defined, as T₂ minus T₁ is less than 200° C. An important aspect of the invention is that a significant portion of the adsorbed CO₂ is desorbed at temperatures below 100° C.

Preferably the gas mixture further comprises water, such that the carbon dioxide/water molar ratio is 1:1 or less.

The gas mixture can be atmospheric air. It may be desirable to pre-dry atmospheric air prior to contacting it with the solid material, to adjust the carbon/dioxide/water molar ratio to within the range of from 1:1 to 1:2.

The adsorption temperature T₁ is preferably less than 40° C., more preferably less than 30° C. The desorption temperature T₂ is preferably less than 120° C., more preferably less than 100° C. For the sake of clarity, T₂ is a range of temperatures at which the desorption reaction takes place. It may be desirable to increase the temperature T₂ in the course of the desorption step. For example, in the case of potassium bicarbonate on activated coal the desorption reaction can be initiated at 40° C., then slowly increased during the desorption step to 180° C. In this example the temperature T₂ is 40 to 180° C., and ΔT is 160° C. or less. A significant portion of the absorbed CO₂ is desorbed at temperatures below 100° C.

The solid material can be purged during the desorption step with an inert gas, such as nitrogen or dry steam.

DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS/EXAMPLES

The following is a description of certain embodiments of the invention, given by way of example only.

Preparation Procedure of Monoliths

Two basic preparation procedures have been employed in the development of the CO₂ absorption monoliths. The first procedure was aimed at creating a wash coat-like layer in the channels of a monolith made of a material that has a low porosity. The second procedure was aimed at impregnating a highly porous monolith material. In the first procedure a suspension of an insoluble inert carrier (for example TiO₂) was created within a solution of a salt for CO₂ absorption (for example K₂CO₃) in demineralized water (“demi water”). The mass ratios of inert:salt:demi water was used in various compositions in the range of 1:1:5 up to 2:1:1. The main determining factor for the demi water content was the pore size of the monoliths to be wash coated (a lower demi water content gives a thicker solution). The suspension was used to wash coat the inert monoliths, after which the monoliths were dried in an oven to remove the demi water.

In the second procedure a solution was prepared of a salt for CO₂ absorption (for example K₂CO₃) in demineralized water. Mass ratios of salt:demi water in the range of 1:1 to 1:20 were used. The porous monoliths were submerged in the solution while taking that no air was trapped within the monoliths. Monoliths were then dried in the oven.

Testing of Monoliths

For the development of carbon dioxide absorbing monoliths a setup was built as schematically depicted in FIG. 1. The general procedure for determining CO₂ adsorption was as follows. The air from which CO₂ is to be absorbed was supplied by an air compressor. From this air compressor the air pressure was reduced, and the air was optionally led through an air pretreatment vessel to remove excess moisture. From this air pretreatment vessel the air flowed through the CO₂ absorber and then out through the vent. After a predetermined exposure time of the CO₂ absorbent to this pretreated airflow the three way valves were switched, and the CO₂ absorber was purged with a controlled flow of nitrogen. Once all the air was removed from the CO₂ absorber, the temperature of the absorption reactor was gradually ramped up in a controlled rate while still being purged with nitrogen. The composition of the outflow of the absorber was monitored by means of a CO₂ analyzer. The total amount of absorbed CO₂ as a function of absorber absorption time and/or air flow rate was calculated by integrating the analyzer signal using the known flow rate of nitrogen.

An example of results of this experimental setup are shown in FIGS. 2 and 3 (these results were obtained with impregnated carbon dioxide absorbing honeycomb monoliths impregnated with K₂CO₃ in 1:10 solution (1 part K₂CO₃ to 10 parts water)). In FIGS. 2 and 3, adsorption was done at ambient temperature, desorption up to 180° C. In FIG. 2 the CO₂ concentration in the outflow of the absorber as measured by the CO₂ analyzer is plotted as a function of the core temperature of the absorber. It can be seen that the release of CO₂ started immediately when heating of the absorbent was commenced. Maximum desorption of CO₂ occurred around 100° C., and high desorption rates occurred between 100° C. and about 140° C. Release of CO₂ ceased around 180° C. What is further seen in FIG. 1 is that the maximum CO₂ concentration desorbed by the monoliths increases when exposure time to air previous to the desorption sequence increases. With an exposure time of 1 hour, the maximum CO₂ concentration desorbed was about 18000 ppm. A 2 hour exposure time resulted in the maximum CO₂ concentration (ppm) desorbed being less than 40000 ppm, about 38500-39000 ppm. With an exposure time of 4 hours, the maximum CO₂ concentration desorbed was about 47000 ppm. With an exposure time of 6 hours, the maximum CO₂ concentration desorbed was about 49000 ppm. With an exposure time of 10 hours, the maximum CO₂ concentration desorbed was about 49500 ppm. With an exposure time of 14 hours, the maximum CO₂ concentration desorbed was about 50000 ppm. With an exposure time of 20 hours, the maximum CO₂ concentration desorbed was just under 50000 ppm, indicating that the exposure time at which most CO₂ is desorbed is above 14 hours.

In FIG. 3 the total amount of CO₂ released as a function of absorbent exposure time to airflow is shown. This is shown for three samples of monoliths: a sample that was not impregnated, a sample that was impregnated with a 1:20 solution of potassium carbonate (by mass) and a sample that was impregnated with a 1:10 solution of potassium carbonate (by mass). It can be seen that unimpregnated samples do not absorb CO₂. The samples impregnated with the 1:10 solution impregnation absorbed more CO₂ than the 1:20 impregnation. Furthermore the absorber appears to be saturated after an exposure to air of approximately 8 hours. After this time the amount of absorbed CO₂ no longer increased.

Application In Greenhouse Agriculture In Arid Areas.

In a specific embodiment, the absorbent material is used for capturing carbon dioxide and water from air in arid regions, such as deserts. As illustrated above in paragraph

even dry air contains more than enough moisture for the adsorption reaction. Moreover, deserts are characterized by large temperature differences between daytime and night time.

Thus, a column filled with absorbent material will absorb water and carbon dioxide from cold desert air during the night. During the day the heat of the sun will raise the temperature of the column high enough to cause the water and carbon dioxide to desorb. Desorbed water and carbon dioxide can be stored in a storage vessel. Both can be used in a greenhouse to supply growing plants with two essential ingredients for the photosynthetic process.

The need for water in this embodiment is greater than the need for carbon dioxide. The column can be partly filled with a desiccant, such as silica gel, and partly with the absorbent material of the invention. The desiccant can be placed upstream or downstream from the CO₂ absorbent.

During the absorption step air can be forced through the column by mechanical means, such as a fan. It is possible also to use the natural temperature differences for creating the required air flow. This is illustrated in FIGS. 4 and 5.

FIG. 4 shows a column 10, containing absorbent material. Column 10 preferably has a rectangular cross section, with a heat capturing surface 11 preferably having an orientation for optimum solar exposure, i.e., a predominantly southern exposure in the northern hemisphere, or a predominantly northern exposure in the southern hemisphere.

Column 10 can be closed at the bottom with bottom plate 12, and at the top with desorption head 13. FIG. 4 shows the column in its open configuration. During the day solar collector 14 captures solar heat, which heats op oil present in solar collector 14. Expansion of the oil caused by the increase in temperature raises the pressure in solar collector 14 and oil lines 15 and 16. The oil pressure is used to move bottom plate 12 and desorption head 13 to their closed positions. At night, the temperature of the oil drops, the oil pressure drops, and bottom plate 12 and desorption head 13 move to their open positions.

FIG. 5 shows, on the left hand side, column 10 in its night time (open) configuration. The rapid drop in ambient temperature causes a downward air flow through column 10, allowing the absorber to absorb water and carbon dioxide from the air.

Column 10 in the right hand portion of FIG. 5 shows the daytime (closed) configuration. Column 10 and its contents are heated up by the sun. This effect is amplified by having heat exchange surface 11 oriented to the sun. Although the ambient temperature rarely exceeds 40° C. (measured in the shade), the temperature of the absorbent material inside column 10 may reach or even exceed 100° C. Water and carbon dioxide absorbed to the absorbent material are desorbed at these temperatures.

Solar heat also causes an upward gas flow inside column 10, so that desorbing gases are collected in desorption head 13, and from there in storage vessel 17.

Storage vessel 17 may be located in a cool place, for example underground. The temperature difference between column 10 and storage vessel 17 reinforces the gas flow. Moreover, a significant portion of the desorbed water collected in storage vessel 17 is condensed to liquid water.

It will be understood that column 10 is not purged when it changes over from absorption mode to desorption mode. As a consequence, storage vessel contains air components, such as nitrogen and oxygen, in addition to carbon dioxide and water. If the contents of storage vessel are to be used in a greenhouse for growing plants, the presence of oxygen and nitrogen is of course not harmful.

The desorption behavior of an exemplary absorbent material was determined in the following experiment. The absorbent material was an active carbon honeycomb, impregnated with K₂CO₃. The material was saturated with carbon dioxide by prolonged exposure to air. Then the material was flushed with nitrogen, while the temperature was increased in steps of 20° C. After each temperature increase the temperature was kept constant until no carbon dioxide was detectable anymore in the nitrogen flow leaving the absorbent bed.

The results are reported in FIG. 6. As can be seen, the desorption predominantly took place at temperatures below 120° C.

Thus, the invention has been described by reference to certain embodiments discussed above. It will be recognized that these embodiments are susceptible to various modifications and alternative forms well known to those of skill in the art. For example, the process may be modified depending on whether the goal is to harvest primarily or exclusively carbon dioxide, or primarily or exclusively water, or a mixture of the two.

Many modifications in addition to those described above may be made to the structures and techniques described herein without departing from the spirit and scope of the invention. Accordingly, although specific embodiments have been described, these are examples only and are not limiting upon the scope of the invention. 

What is claimed is:
 1. A solid material capable of reversibly adsorbing carbon dioxide, said solid material comprising a porous carrier material having deposited thereon: (i) a salt capable of reacting with carbon dioxide; and optionally (ii) a particulate, water-insoluble inorganic material.
 2. The solid material of claim 1 wherein the salt is capable of reacting with carbon dioxide and water to form a bicarbonate.
 3. The solid material of claim 1 or 2 which is capable of adsorbing carbon dioxide at a first temperature T₁ and of desorbing carbon dioxide at a second temperature T₂, such that T₂>T₁ and ΔT, defined, as T₂ minus T₁ is less than 200° C., preferably less than 160° C.
 4. The solid material of claim 3 characterized by a desorption of carbon dioxide which takes place to a significant extent at a temperature below 100° C.
 5. The solid material of any one of claims 2 through 4 wherein the salt capable of reacting with water and carbon dioxide to form a bicarbonate is a salt of an alkali metal or an alkaline earth metal.
 6. The solid material of claim 5 wherein the salt capable of reacting with water and carbon dioxide to form a bicarbonate is a salt of Li, Na, K, Ca, Ba, or a mixture thereof.
 7. The solid material of any one of claims 2 though 5 wherein the salt capable of reacting with water and carbon dioxide to form a bicarbonate comprises a carbonate.
 8. The solid material of claim 7 wherein the salt capable of reacting with water and carbon dioxide to form a bicarbonate comprises potassium carbonate.
 9. The solid material of any one of claims 1 though 7 wherein the porous carrier material is a honeycomb monolith material.
 10. The solid material of claim 9 wherein the honeycomb monolith material is made of a ceramic material, zeolite, or activated carbon.
 11. The solid material of claim 9 wherein the porous carrier material is made of a ceramic foam.
 12. The solid material of any one of claims 1 through 11 wherein the porous carrier is made of activated carbon having a specific surface area in the range of from 150 m²/g to 600 m²/g.
 13. The solid material of any one of claims 1 through 12 wherein the particulate, water-insoluble inorganic material is an inorganic oxide having a specific surface area of less than 100 m²/g.
 14. The solid material of claim 13 wherein the specific surface area of the particulate, water-insoluble inorganic material is adjusted by calcination or steam calcination.
 15. The solid material of claim 11 or 12 wherein the particulate water-insoluble inorganic material is selected from the group consisting of alumina, silica, titania, zirconia, ceria, clay, zeolite, layered hydroxide material, hydrotalcite, and mixtures thereof.
 16. The solid material of claim 15 wherein the particulate water-insoluble inorganic material is titania.
 17. A process for recovering carbon dioxide from a carbon dioxide containing gas mixture comprising (a) at a first temperature T₁, contacting the carbon dioxide containing gas mixture with a solid material capable of reversibly adsorbing carbon dioxide according to any one of the preceding claims; and (b) desorbing carbon dioxide from the solid material at a temperature T₂, such that T₂>T₁ and ΔT, defined, as T₂ minus T₁ is less than 200° C., preferably less than 160° C.
 18. The process of claim 17 wherein the gas mixture further comprises water, such that the carbon dioxide/water molar ratio is 1:1 or less.
 19. The process of claim 17 or 18 wherein the gas mixture is atmospheric air.
 20. The process of claim 19 wherein the atmospheric air is pre-dried to adjust the carbon dioxide/water molar ratio to within the range of from 1:1 to 1:2.
 21. The process of any one of claims 17 through 20, wherein T₁ is less than 40° C., preferably less than 30° C.
 22. The process of any one of claims 17 through 21 wherein T₂ is less than 120° C., preferably less than 100° C.
 23. The process of any one of claims 17 through 22 wherein T₁ is ambient, and T₂ is less than 100° C.
 24. The process of any one of claims 17 through 23 wherein the solid material is purged during step (b) with a flow of a gas that does not react with the salt.
 25. The process according to any one of claims 17 through 24 used for collecting carbon dioxide and water from desert air for use in growing plants.
 26. The process of claim 25 wherein naturally occurring temperature differences are used to achieve the absorption temperature T₁ and the desorption temperature T₂.
 27. The process of claim 26 wherein naturally occurring temperature differences are used to achieve gas flows through a bed of the solid material. 